The present invention relates to the field of gas furnaces, and in particular to a pressure switch assembly for a multistage gas furnace.
With a furnace for heating a residential or commercial space, a thermostat senses when the temperature of an interior comfort space is below a set temperature. When the temperature drops below the set temperature, the thermostat provides a call for heat that turns on a gas burner and, after a delay time, a circulation air blower. The gas burner injects flame and heated gas into a heat exchanger, which heats the circulation air that is then returned to the interior space. An induced combustion fan draws combustion gases through the heat exchanger and exhausts them into a vent pipe for discharge to an outside environment. Heating continues until the thermostat senses that the interior room air has been heated above the set point, at which time it opens and ends the call for heat.
Multi-stage furnaces have gas burners that operate at different flow rates, ranging from a high flow rate (i.e., high fire) to varying levels of partial flow rates. The high fire mode is employed when there is a high demand for heating, such as when the partial flow rates fail to increase the interior room air temperature above the set point in an allotted time or when specifically commanded by the thermostat. The partial flow rates are employed when there is a lower demand for heat, and the gas burners provide a corresponding level of fire proportionate to the demand for heat.
The gas burners can be actuated into the various flow rate modes based on the states of combustion pressure switches in the furnace. Combustion pressure switches, which sense the negative pressure in the furnace combustion chamber, serve to turn the burners on only if the inducer fan is bringing enough combustion air in to support the level of fire provided by the burners. In conventional furnace systems, the furnace control is designed to have the same number of pressure switch inputs as the number of operating modes supported. Thus, a change in the number of operating modes in the furnace typically requires a change to the control circuitry of the furnace.